1. Field of the Invention
The invention relates to a surface texture measuring instrument, and a surface texture measurement method, in which the contour, surface roughness, surface waviness, etc., of a workpiece is measured by moving a stylus along a surface of the workpiece, in particular, relates to a technique for correcting the error due to the tip shape of the stylus. The invention also relates to a stylus radius measuring instrument.
2. Description of the Related Art
Surface texture measuring instruments, in which a stylus is made to contact a workpiece and moved along the workpiece surface to measure the contour, surface roughness, surface waviness, etc., of the workpiece, have been known in the related art. Such surface texture measuring instruments are used in the measurement of machined parts, etc. With a surface texture measuring instrument, when the stylus is moved in one direction (X axis) along the workpiece surface by means of a motor, the stylus becomes displaced in the vertical direction (Z direction) due to surface irregularity of the workpiece surface. By measuring the displacement in the X direction and the displacement in the Z direction, the contour, surface roughness and surface waviness of the workpiece can be measured.
However, since the cross-sectional shape of the tip portion of the stylus that comes in contact with the workpiece is not a point but is of an arcuate shape of finite size, the locus expressed by the Z-direction displacement of the stylus will not coincide with the actual contour of the workpiece.
FIG. 8 illustrates the difference between the locus obtained by measurement and the actual contour of a workpiece. In this Figure, a stylus 1 is displaced in the X direction and is displaced in the Z direction according to the surface texture of the workpiece. As indicated by 100 in the Figure, the measured locus of the workpiece that corresponds to the displacement of the stylus 1 is the locus of a predetermined reference point of the stylus 1 (approximately the central point of the arcuate portion of the stylus tip). In the measured locus of the workpiece, an offset, corresponding to the radius r of the tip arc of stylus 1, arises with respect to the actual contour 200 of the workpiece. In the case where the stylus is mounted to a rotatable arm and undergoes arcuate movement, the locus 100 is obtained by correcting for the arc error that accompanies this arcuate movement. Thus in order to obtain the actual contour of the workpiece, the measured value was offset by just the radius r of the tip arc of stylus 1.
However, the cross-sectional shape of the tip of stylus 1 is not strictly circular but its radius value differs according to position. Thus as the workpieces to be measured are made finer and higher in accuracy, cases have arisen where the use of just the representative value of the tip radius r as the offset in the related art is inadequate for measurements of such high accuracy.
The invention has been made in view of the above problem of the related art. It is an object of the invention to provide a surface texture measuring instrument and a surface texture measuring method, in which the actual surface texture, such as the contour, surface roughness, surface waviness, etc., of a workpiece can be obtained at high accuracy by performing correction, based on the cross-sectional shape of the tip of the stylus, of the measured value obtained by measuring the displacement of the stylus in the Z direction. It is another object of the invention to provide a stylus radius measuring instrument.
In order to achieve the above object, a surface texture measuring instrument according to the invention, in which a surface texture of a workpiece is measured by moving a stylus along a surface of the workpiece and measuring a displacement of the stylus in the Z direction, comprises a memory section, which stores radius values according to angle of the stylus that are obtained by measuring a spherical, cylindrical, or knife-edge-shaped reference gage of known radius with the stylus, and a correction section, which corrects measured values based on the radius values of the stylus stored in the memory section.
Here, in the case where the stylus is mounted on a rotatable arm and undergoes arcuate movement, the surface texture measuring instrument preferably comprises a detection section, which detects an angle of inclination of the stylus with respect to the Z direction, wherein the correction section corrects the measured values based on the radius values and the angles of inclination of the stylus.
The object van be also achieved by a surface texture measuring instrument, in which a surface texture of a workpiece is measured by moving a stylus along a surface of the workpiece and measuring a displacement of the stylus in the Z direction, comprising a radius value calculation section, which calculates radius values according to angle of the stylus that are obtained by measuring a spherical, cylindrical, or knife-edge-shaped reference gage of known radius with the stylus, memory section, which stores the radius values of the stylus, and a correction section, which corrects measured values based on the radius values of the stylus stored in the memory section.
Here, in the case where the stylus is mounted on a rotatable arm and undergoes arcuate movement, the surface texture measuring instrument preferably comprises a detection section, which detects an angle of inclination of the stylus with respect to the Z direction, wherein the radius calculation section calculates the radius values based on the angles of inclination of the stylus, and the correction section corrects the measured values based on the radius values and the angles of inclination of the stylus.
Further, in order to achieve the above object, a surface texture measurement method according to the invention, in which a surface texture of a workpiece is measured by moving a stylus along a surface of the workpiece and measuring a displacement of the stylus in the Z direction, comprises a measurement step, in which the workpiece is measured by the stylus, and a correction step, in which measured values of the workpiece are corrected using correction values. The correction value is the difference between measured value according to angle that was obtained by measuring a spherical, cylindrical, or knife-edge-shaped reference gage of known radius with the stylus and the radius value of the reference gage.
Here, in the case where the stylus is mounted on a rotatable arm and undergoes arcuate movement, the measured value is preferably corrected in the correction step by using the correction values and angles of inclination of the stylus with respect to the Z direction.
The object can be achieved by a surface texture measurement method, in which the surface texture of a workpiece is measured by moving a stylus along a surface of the workpiece and measuring a displacement of the stylus in the Z direction, comprising a measurement step, in which the workpiece is measured by the stylus, a correction value calculation step, in which correction values are calculated as the difference between measured values according to angle that were obtained by measuring a spherical, cylindrical, or knife-edge-shaped reference gage of known radius with the stylus and the radius value of the reference gage, and a correction step, in which measured values of the workpiece are corrected using the correction values.
Here, in the case where the stylus is mounted on a rotatable arm and undergoes arcuate movement, the correction values are preferably corrected in the correction value calculation step by using angles of inclination of the stylus with respect to the Z direction, and the measured values are corrected in the correction step by using the correction values and the angles of inclination of the stylus.
As has been described above, with this invention, a spherical, cylindrical, or knife-edge-shaped reference gage is measured and the radius values according to angle of the stylus are calculated from the measurement results of the reference gage. To be more specific, radius values of the stylus are calculated by subtracting the known radius values of the reference gage from the measured values of the reference gage. The radius values according to angle of the stylus indicate the deviation of the stylus from a perfect circle, and by using these radius values as correction values and correcting the measured values obtained by actual movement of a workpiece by the correction values according to angle, the actual contour of the workpiece can be obtained.